Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly, to a method for manufacturing a less than 40 .ANG. thick gate silicon oxide layer used in a metal oxide semiconductor (MOS) transistor or the like.
In a prior art method for manufacturing a MOS transistor or the like where a silicon substrate is loaded in an oxidation furnace whose temperature is about 300 to 700.degree. C., the temperature of the oxidation furnace is raised to 750 to 900.degree. C., and an oxidation operation is performed upon the silicon substrate to grow an essential silicon oxide layer on the silicon. In this case, an initial silicon oxide layer is grown before the growth of the essential silicon oxide layer. That is, in order to grow the initial silicon oxide layer, a mixture gas of 1 to 2 percent oxygen (O.sub.2) and nitrogen is supplied to the furnace during the silicon substrate loading step and the temperature raising step (see: JP-A-4-186835). This will be explained later in detail.
Note that an entire gate silicon oxide layer is formed by the initial silicon oxide layer plus the essential silicon oxide layer.
Although the initial silicon oxide layer is poor in terms of quality, the initial silicon oxide layer serves as a cap layer for preventing the silicon substrate from migrating at about 750 to 900.degree. C. where the essential silicon oxide layer is grown. Therefore, the initial silicon oxide layer suppresses the roughness of the surface of the silicon substrate, so as to obtain a favorable interface between the silicon substrate and the gate silicon oxide layer, which would improve the reliability of the gate silicon oxide layer.
In the gate silicon oxide layer, however, since the initial silicon oxide layer is grown in both of the silicon substrate loading step and the temperature raising step where the concentration of oxygen flowing through the furnace is high, i.e., 1 to 2 percent, when the gate silicon oxide layer is too thin, for example, less than about 40 .ANG., the ratio of the initial silicon oxide layer to the entire gate silicon oxide layer becomes larger than about 50 percent, which would deteriorate the entire gate silicon oxide layer.